Ryanair allows check-ins 30 days before departure. Easyjet, Wizzair follow suit.
Why is that? Could it be that low costers being relatively new have better IT systems?
"Historically" - meaning about 10-15 years ago, when mobile boarding passes weren't a thing - (european) LCCs required travellers to print the boarding pass before flying or pay extra for check-in at the airport. I can imagine that one reason for them to open online check-in rather early those days, was to allow the average "vacation traveller" to print both boarding passes for outgoing and return flight at home, before heading out on vacation.
Seeing how a lot of people still prefer printed boarding passes - because scanning a code off their smartphone never works, because they are afraid of the battery running low, because handling paper is less of a hassle, because they don't have a smartphone, ... - I think they just never changed that rule, though I have no sources to back up that assumption.
Ryanair actually allows 'check-in' 60 days in advance, but that option is only a way to lure more money from their customers. In reality, 'check-in' more than 24 hours before departure is only possible if you either have a flexi plus ticket (which includes this option) or pay extra to select a seat and does not mean anything more than that you can actually select which seat you will be sitting in.
Without having an expensive flexi plus ticket or paying a premium for seat selection, 'regular' travellers can also with Ryanair not check-in more than 24 hours before the flight and will in that case only be given a random seat. Passengers travelling together will not be assigned seats together unless they pay for the seat selection.
